simple: take the .45 acp and update it to modern ballistics by giving it thicker brass to withstand higher pressures. This is the same basic process used to upgrade the .38 acp to the .38 Super cartridge.
Here is what Wikipedia has to say about it:
The .45 Super is a large and powerful smokeless powder center fire metallic firearm cartridge developed in 1988 by Dean Grennell, a well-known writer in the firearms field as well as managing editor of Gun World magazine. Born of a desire to update and improve the 1906 era .45 ACP, the .45 Super cartridge is dimensionally identical with the older .45 ACP round. A notable difference is that it has a thicker case wall than its ACP brethren cartridge, and the Super is loaded to higher pressures, which makes it a substantially more powerful round than the standard .45 ACP. It offers an average 300 foot per second (ft/s) improvement in muzzle velocity. The cartridge was co-developed by Tom Fergerson and Ace Hindman.
In 1988, a Gun World article detailed Grennell's efforts to update the .45 ACP for the 21st Century, a difficult endeavor due to the inherent design limitations of the veteran round. Introduced in the early 20th Century, the .45 ACP has a relatively large case capacity which was dictated by the relatively low pressure powders in use at the time of its development; as a result, it operates in the modest range of 19,900 – 22,000 Copper units of pressure (CUP). In contrast, current day cartridges using modern nitrocellulose powders generating higher pressure can produce a CUP in the 28,000 – 39,000 range. As it was originally designed for lower pressures, the .45 ACP case has relatively thin walls and weak case head and web specifications; it cannot reliably contain increased pressures. The layout of most M1911 pistols' chambers presents yet another challenge in that the case head is not fully supported in the cartridge feed ramp area; pushing the envelope in this critical area with too much pressure risks a catastrophic failure, resulting in a case bursting in the chamber. To rule out such a dangerous possibility, Grennell chose to use brass formed from the stronger and more modern .451 Detonics, shortened to the overall length of the .45 ACP design. Support for the case head was also addressed by adopting a new chamber and barrel design which supports the base area of the case. Other areas of the model 1911 pistol design were also strengthened, including the addition of a heavier recoil spring and a strengthened firing pin redesigned to prevent primer material from flowing into the firing pin channel under high chamber pressures.
Manufacturers such as Heckler & Koch (USP and HK45), and Springfield Armory, Inc. currently offer pistols rated to fire .45 Super ‘out of the box’. Although they will chamber, the firing of .45 Super rounds in non-rated standard .45 ACP automatics is not recommended, as doing so risks a case failure in the unsupported chamber and at the very least would batter the slide and almost certainly shorten the life of the pistol.
The .450 SMC is a variant of the .45 Super with a smaller primer pocket, which is touted as being stronger due to having more brass in the web area.
A number of bullet weight and velocity combinations are offered in .45 Super, including a 185-grain (12.0 g) bullet propelled at 1,300 ft/s, a 200-grain (13 g) at 1,200 ft/s, and a 230-grain (15 g) at 1,100 ft/s. as well as other weight/velocities provided by Super Express cartridges and Buffalo Bore, such as 255grain at 1050 ft/s.
Sedalia, Missouri based Starline Brass company eventually began marketing factory manufactured brass cases for the chambering, taking the round out of the obscure wildcat cartridge realm. In addition, Ace Custom .45’s Inc. of Cleveland, Texas trademarked the .45 Super name in 1994 and currently markets factory .45 Super pistols, as well as gunsmith adaptations of .45 ACP pistols, and .45 ACP conversion kits. Texas Ammunition and Buffalo Bore offer factory loaded ammunition which is marketed by Ace Custom and others. The Dan Wesson 460 Rowland will also chamber a .45 Super.
RealGuns.com 45 Super Conversion
Data from the defunct Triton Site
What is the 45 Super?
How to convert a Glock 21 to 45 Super
* ".45 SuperŪ" is a Trade Mark of Ace Custom 45's Inc.